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The chemical composition of nutrient-saturated cultures of Emiliania huxleyi, Amphidinium carterae, and Staurastrum luetkemuelleri was studied. The variation in chemical composition of natural phytoplankton communities in the North Sea, the Trondheimsfjord, and a eutrophic lake was also studied. Nutrient status was evaluated by measurement of the algal protein/carbohydrate, N/C, P/C, and N/P ratios. Tests for P-deficiency were carried out by measuring the increase in ATP upon addition of phosphate. At saturation the N/C ratio was ˜0.14 in marine species and ˜0.05 in Staurastrum. Saturation P/C ratios (excluding polyphosphates) were species-dependent, ranging from 0.017 (Skeletonema) to 0.006 (Amphidinium). Amphidinium and Staurastrum store polyphosphates when grown in P-rich media; true marine planktonic species do not. Natural communities tended to be close to nutrient saturation at low biomass densities and nutrient deficient at high densities. In the North Sea, nitrogen was clearly limiting. In waters off the Møre coast and in the Trondheimsfjord, growth was nearly balanced with respect to N and P at high salinities (>25‰) and clearly P-limited in brackish fjord waters. In dense communities, the N/P ratio was inversely related to salinity. Freshwater communities were clearly P-limited, but responses were dampened when daphnia or whitefish were introduced, due to increased excretion of nutrients.
Sakshaug et al. (Sat,) studied this question.